Printers and copiers have in recent years been making the transition from analog to digital, and, while there is strong demand for an excellent latent image reproducibility and a high resolution, there is at the same time strong demand for greater energy savings and downsizing, particularly with regard to printers.
Simplifying the fixing unit and developing assembly (cartridge) is effective for getting greater energy savings to coexist in balance with downsizing. Film fixing is an example of a fixing unit that facilitates simplification of the heat source and structure. In this fixing method, fixing is carried out while bringing the recording medium into close contact with the heating element through the intermediary of a fixing film, and as a result an excellent thermal efficiency is obtained during melt adhesion of the toner on the recording medium.
However, in order to achieve even more substantial energy savings, the development is required of systems and materials that enable a lowering of the amount of heat from the heating element and fixing at low temperatures. During fixing in film fixing methods the film and recording medium are brought into close contact by a contacting pressure member, but since a strong pressure is not applied, the fixing characteristics in particular of the toner must be substantially improved. That is, the low-temperature fixability of the toner must be improved.
As a general matter, efforts to improve the low-temperature fixability frequently also result in a lowering of the storage stability of the toner in a high-temperature environment. For example, when a toner composition that softens at lower temperatures is used, the toner may undergo blocking in a high-temperature environment and a stable image density may not be obtained. It has thus been quite difficult to have the low-temperature fixability coexist in balance with the storage stability.
Controlling the properties of the binder resin in the toner particle core is known as a technique for improving the low-temperature fixability. In Patent Literature 1, the ratio between the high-molecular weight component and low-molecular weight component in the toner is controlled and the flow tester-measured softening temperature of the toner and softening temperature of the binder resin are controlled. However, when the amount of high-molecular weight component is controlled in a broad range of from at least 15% to not more than 50 mass % and the softening temperature of the toner is not more than 150° C., fixing at low temperature • light pressure is thought to be strongly impaired since the controlled temperature range is a high temperature region. Otherwise, the molecular weight of the binder resin, the softening temperature of the toner, the melting temperature of the toner by the ½ method (referred to below as the “softening point”), and the glass-transition temperature of the toner are controlled in Patent Literature 2. However, issues remain with high-speed fixing using a hydrocarbon wax with a high melting point as the release agent, and, in addition, since the softening temperature is low, there is room for improvement from the perspective of a balanced coexistence with the storage stability of the toner.
On the other hand, the use of an external additive to inhibit blocking is known as a technique for improving the storage stability. The exposure of the toner particle core can be suppressed, and the blocking resistance can then be improved, by covering the toner particle with an external additive. However, external additives impede fixing because they interfere with thermal conduction to the toner particle, and as a consequence it is quite difficult to bring about a high degree of coexistence between the low-temperature fixability and the storage stability just by coverage with an external additive alone.
Patent Literature 3 states that—by using two types of silica fine particles (number-average primary particle diameter is at least 25 nm, and at least 45 nm) with different particle diameters as the external additive—the storage stability can be maintained even at a low coverage ratio of the toner particle by the silica fine particles and the impediment to fixing can also be suppressed. However, no specific evaluation of the fixing performance is mentioned and the effect on the fixing performance is thus unclear; in addition, due to the low coverage ratio, questions remain as to whether the storage stability can be maintained when an easily softened toner particle core is used.